Wednesday, February 27, 2013

Virtual Photons Become Real in a Vacuum

The zero-point energy stored in the modes of an electromagnetic cavity
has experimentally detectable effects, giving rise
to an attractive interaction between the
opposite walls, the static Casimir effect. A dynamical version of this
effect was
predicted to occur when the vacuum energy
is changed either by moving the walls of the cavity or by changing the
index of
refraction, resulting in the conversion of
vacuum fluctuations into real photons. Here, we demonstrate the
dynamical Casimir
effect using a Josephson metamaterial
embedded in a microwave cavity at 5.4 GHz. We modulate the effective
length of the cavity
by flux-biasing the metamaterial based on
superconducting quantum interference devices (SQUIDs), which results in
variation
of a few percentage points in the speed of
light. We extract the full 4 × 4 covariance matrix of the emitted
microwave radiation,
demonstrating that photons at frequencies
symmetrical with respect to half of the modulation frequency are
generated in pairs.
At large detunings of the cavity from half
of the modulation frequency, we find power spectra that clearly show
the theoretically
predicted hallmark of the Casimir effect: a
bimodal, “sparrow-tail” structure. The observed substantial photon flux
cannot
be assigned to parametric amplification of
thermal fluctuations; its creation is a direct consequence of the
noncommutativity
structure of quantum field theory. See: Dynamical Casimir effect in a Josephson metamaterial